Diagnostic strategy for an electric motor using sensorless control and a position sensor

ABSTRACT

A strategy to control and diagnose the operation of an electric motor using a sensorless control system augmented by feedback from a position and speed sensor is disclosed. The strategy can improve the robustness of operation and diagnose potential faults in electric motors. The present invention includes a method for diagnosing operation of an electric motor and a method and system for controlling an electric motor. In the diagnostic method, a sensorless system and a sensor based system are checked against each other to determine if either of the systems is faulted. In the control method, the sensor based control system is used when the motor speed is below a predetermined threshold and the sensorless system is used when the motor speed is above the predetermined threshold. The position sensor can be a low resolution position sensor, an engine crankshaft sensor, an engine camshaft sensor, or a transmission sensor.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a division of U.S. application Ser. No.10/065,026, filed Sep. 11, 2002. U.S. application Ser. No. 10/065,026 ishereby incorporated in its entirety.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to an electricallypowered vehicle, such as an electric vehicle (EV), a hybrid electricvehicle (HEV) or a fuel cell vehicle (FCV). More specifically theinvention relates to a strategy to diagnose a potential fault in anelectric motor. The present invention an improve the robustness ofoperation and diagnose potential faults in electric motors by utilizinga sensorless control scheme augmented by feedback from a low-resolutionposition and speed sensor.

[0004] 2. Background of the Invention

[0005] The need to reduce fossil fuel consumption and emissions inautomobiles and other vehicles predominately powered by internalcombustion engines (ICEs) is well known. Vehicles powered by electricmotors attempt to address these needs. Another alternative solution isto combine a smaller ICE with electric motors into one vehicle. Suchvehicles combine the advantages of an ICE vehicle and an electricvehicle and are typically called hybrid electric vehicles (HEVs). Seegenerally, U.S. Pat. No. 5,343,970 to Severinsky.

[0006] The HEV is described in a variety of configurations. Many HEVpatents disclose systems where an operator is required to select betweenelectric and internal combustion operation. In other configurations, theelectric motor drives one set of wheels and the ICE drives a differentset.

[0007] Other, more useful, configurations have developed. For example, aseries hybrid electric vehicle (SHEV) configuration is a vehicle with anengine (most typically an ICE) connected to an electric motor called agenerator. The generator, in turn, provides electricity to a battery andanother motor, called a traction motor. In the SHEV, the traction motoris the sole source of wheel torque. There is no mechanical connectionbetween the engine and the drive wheels. A parallel hybrid electricalvehicle (PHEV) configuration has an engine (most typically an ICE) andan electric motor that work together in varying degrees to provide thenecessary wheel torque to drive the vehicle. Additionally, in the PHEVconfiguration, the motor can be used as a generator to charge thebattery from the power produced by the ICE.

[0008] A parallel/series hybrid electric vehicle (PSHEV) hascharacteristics of both PHEV and SHEV configurations and is sometimesreferred to as a parallel/series “split” configuration. In one ofseveral types of PSHEV configurations, the ICE is mechanically coupledto two electric motors in a planetary gear-set transaxle. A firstelectric motor, the generator, is connected to a sun gear. The ICE isconnected to a carrier gear. A second electric motor, a traction motor,is connected to a ring (output) gear via additional gearing in atransaxle. Engine torque can power the generator to charge the battery.The generator can also contribute to the necessary wheel (output shaft)torque if the system has a one-way clutch. The traction motor is used tocontribute wheel torque and to recover braking energy to charge thebattery. In this configuration, the generator can selectively provide areaction torque that may be used to control engine speed. In fact, theengine, generator motor and traction motor can provide a continuousvariable transmission (CVT) effect. Further, the HEV presents anopportunity to better control engine idle speed over conventionalvehicles by using the generator to control engine speed.

[0009] The desirability of combining an ICE with electric motors isclear. There is great potential for reducing vehicle fuel consumptionand emissions with no appreciable loss of vehicle performance ordriveability. The HEV allows the use of smaller engines, regenerativebraking, electric boost, and even operating the vehicle with the engineshut down. Nevertheless, new ways must be developed to optimize theHEV's potential benefits.

[0010] One such area of development is diagnosing potential faults in anelectric motor and increasing the robustness of the operation of anelectric motor. An effective and successful HEV design (or any vehiclepropelled by electric motors) requires reliable operation. Reliableoperation can be improved through careful diagnosis of potential faultswithin the electric motor and increasing the robustness of electricmotor operation. Thus there is a need for a strategy to effectivelydiagnose potential faults in an electric motor propelled vehicle'selectrical motor and increase the robustness of electric motoroperation. One strategy to improve the robustness of operation anddiagnose potential faults in electric motors is to utilize a sensorlesscontrol scheme coupled with feedback from a low-resolution position andspeed sensor.

[0011] Sensorless control schemes for electric machines (also referredto as electric motors or generators) are known in the art. Electricmachines can be induction, synchronous or switched reluctance type. Forexample, U.S. Pat. No. 6,137,258 tojansen describes a system forspeed-sensorless control of an induction machine (electric motor) thatincludes a flux regulator and torque current calculator for operatingthe machine in a saturated state. U.S. Pat. No. 6,163,119 to jeongdescribes a sensorless speed control method for a high speed motor thatutilizes a reverse electromotive force. U.S. Pat. No. 5,920,175 to Joneset al. describes a sensorless control system for operating an invertercoupled to a switched reluctance machine that includes an instantaneousposition generation circuit that develops a signal for controllingcommutation of the switched reluctance machine. See also, U.S. Pat. No.5,811,957 to Bose et al., and U.S. Pat. No. 6,104,113 to Beifus.

[0012] Low resolution shaft position and speed sensors are also known inthe art and are commonly installed in automotive vehicles. Crankshaftposition and speed sensors, camshaft position and speed sensors andtransmission position and speed sensors are examples of low resolutionshaft position sensors used in automotive vehicles.

[0013] However, sensorless control schemes for electric motors and lowresolution position and speed sensors each have their drawbacks andlimitations. Sensorless control schemes often fail at low shaftrotational speeds and thus often limited to high shaft rotationalspeeds. Low resolution shaft position and speed sensors can measureshaft position and speed at low shaft rotational speeds, but havelimited accuracy.

SUMMARY OF INVENTION

[0014] Accordingly, the present invention provides a strategy to controlan electric motor using a sensorless control scheme augmented byfeedback from a low-resolution position and speed sensor. The strategycan improve the robustness of operation and diagnose potential faults inelectric motors.

[0015] In accordance with an important aspect of the invention, twodifferent control systems are utilized in order to optimize motorperformance. A low resolution position and speed sensor is used at lowmotor speeds while a sensorless control system is used at higher motorspeeds, i.e., those which exceed a preselected threshold. In accordancewith a related aspect, the invention provides a method for diagnosingoperation of an electric motor utilizing two such different controlsystems.

[0016] In accordance with a further aspect of the invention, an apparentshaft position of a motor based on data provided by one of the differentcontrol systems can be corrected using data provided by the other ofsaid control systems.

[0017] In accordance with yet further aspects of the invention, controlsystems and automotive vehicles embodying the two different controlsystems are provided.

[0018] The present invention includes a method for diagnosing operationof an electric motor that comprises the steps of:

[0019] determining a first shaft position using a sensorless controlsystem; determining a second shaft position using a position sensor; andevaluating operation of the electric motor based at least in part ondata output related to the first shaft position and to the second shaftposition. The diagnostic method can also include evaluating operation ofthe sensorless control scheme based on the second shaft position, aswell as evaluating operation of the position sensor based on the firstshaft position, and determining a modified shaft position based on thefirst shaft position and the second shaft position.

[0020] The present invention also includes a method for controlling anelectric motor that comprises the steps of: determining an electricmotor rotational speed; operating the electric motor using a sensorlesscontrol system if the electric motor rotational speed is above apredetermined threshold; and operating the electric motor using a sensorbased control system if the electric motor rotational speed is below thepredetermined threshold. The control method can also include correctingthe sensorless control system using data output from the sensor basedcontrol system and correcting the sensor based control system using dataoutput from the sensorless control system.

[0021] Operating the electric motor using the sensorless control systemif the electric motor rotational speed is above a predeterminedthreshold can include, alternatively, the steps of: determining motorspeed and position from a plurality of phase current and phase voltagesignals; determining an inverter voltage command from the motor speedand position; determining the plurality of phase current and phasevoltage signals from the inverter voltage command; determining motorspeed and position from a position sensor; and correcting the phasecurrent and phase voltage signal determined motor speed and positionwith the position sensor determined motor speed and position.

[0022] Operating the electric motor using a sensor based control systemif the electric motor rotational speed is below the predeterminedthreshold step can be based on: determining motor speed and positionfrom a position sensor; determining an inverter voltage command from themotor speed and position; determining a plurality of phase current andphase voltage signals from the inverter voltage command; and determiningmotor speed and position from a plurality of phase current and phasevoltage signals and correcting the position sensor determined motorspeed and position with the phase current and phase voltage signaldetermined motor speed and position data.

[0023] The present invention also includes a system to control anelectric motor comprising: an inverter operatively connected to themotor; a position estimator operatively connected to the motor and theinverter; a torque controller operatively connected to the positionestimator and the inverter; a position sensor operatively connected tothe motor and the position estimator; means for determining a firstelectric motor shaft position based on an output from the inverter;means for determining a second electric motor shaft position based on anoutput from the position sensor; and means for correcting the firstelectric motor shaft position using data about the second electric motorshaft position. The control system can also include means for correctingthe second electric motor shaft position using data related to the firstelectric motor shaft position. The position sensor can be a lowresolution position sensor, an engine crank shaft position sensor, anengine camshaft position sensor, or a transmission sensor.

[0024] Other objects of the present invention will become more apparentto persons having ordinary skill in the art to which the presentinvention pertains from the following description taken in conjunctionwith the accompanying figures.

BRIEF DESCRIPTION OF DRAWINGS

[0025] The foregoing objects, advantages, and features, as well as otherobjects and advantages, will become apparent with reference to thedescription and figures below, in which like numerals represent likeelements and in which:

[0026]FIG. 1 illustrates a general hybrid electric vehicle (HEV)configuration.

[0027]FIG. 2 illustrates a sensorless control scheme for an electricmotor augmented by feedback from a low resolution position and speedsensor.

DETAILED DESCRIPTION

[0028] The present invention relates to electric motors. As the use ofelectric motors in vehicle applications increases, robust motoroperation and diagnosing potential faults is increasingly moreimportant. This is especially true in the harsh conditions typicallyexperienced by motors used as vehicle components. For demonstrationpurposes and to assist in understanding the present invention, it isdescribed in a hybrid electric vehicle (HEV) application. FIG. 1demonstrates just one possible HEV configuration, specifically aparallel/series hybrid electric vehicle (powersplit) configuration.

[0029] In a basic HEV, a planetary gear set 20 mechanically couples acarrier gear 22 to an engine 24 via a one-way clutch 26. The planetarygear set 20 also mechanically couples a sun gear 28 to a generator motor30 and a ring (output) gear 32. The generator motor 30 also mechanicallylinks to a generator brake 34 and is electrically linked to a battery36. A traction motor 38 is mechanically coupled to the ring gear 32 ofthe planetary gear set 20 via a second gear set 40 and is electricallylinked to the battery 36. The ring gear 32 of the planetary gear set 20and the traction motor 38 are mechanically coupled to drive wheels 42via an output shaft 44.

[0030] The planetary gear set 20, splits the engine 24 output energyinto a series path from the engine 24 to the generator motor 30 and aparallel path from the engine 24 to the drive wheels 42. Engine 24 speedcan be controlled by varying the split to the series path whilemaintaining the mechanical connection through the parallel path. Thetraction motor 38 augments the engine 24 power to the drive wheels 42 onthe parallel path through the second gear set 40. The traction motor 38also provides the opportunity to use energy directly from the seriespath, essentially running off power created by the generator motor 30.This reduces losses associated with converting energy into and out ofchemical energy in the battery 36 and allows all engine 24 energy, minusconversion losses, to reach the drive wheels 42.

[0031] A vehicle system controller (VSC) 46 controls many components inthis HEV configuration by connecting to each component's controller. Anengine control unit (ECU) 48 connects to the engine 24 via a hardwireinterface. All vehicle controllers can be physically combined in anycombination or can stand as separate units. They are described asseparate units here because they each have distinct functionality. TheVSC 46 communicates with the ECU 48, as well as a battery control unit(BCU) 50 and a transaxle management unit (TMU) 52 through acommunication network such as a controller area network (CAN) 54. TheBCU 50 connects to the battery 36 via a hardwire interface.

[0032] The TMU 52 controls the generator motor 30 and traction motor 38via a hardwire interface to a generator motor control unit (GMCU) 56 anda traction motor control unit (TMCU) 58. The control units 46, 48, 50,52, 56, and 58 and CAN 54 can include one or more microprocessors,computers, or central processing units; one or more computer readablestorage devices; one or more memory management units; and one or moreinput/output devices for communicating with various sensors, actuatorsand control circuits.

[0033] The present invention is a strategy to control an electric motorusing a sensorless control scheme augmented by feedback from alow-resolution position and speed sensor. This invention can be in acomputer readable format embodied in one or more of the computingdevices described above.

[0034]FIG. 2 illustrates a sensorless control scheme for an electricmotor augmented by feedback from a low resolution position and speedsensor. The TMCU 58 is shown and is communicatively connected to aninverter 80, a torque controller 82 and a position estimator 84.Inverters, known in the art, change direct current into alternatingcurrent having the appropriate number of phases. Inverter 80 is shown asa three-phase inverter. The current and voltage signals of the threephases are used to drive the traction motor 38. The current and voltagesignals of the three phases are also input into the torque controller 82and the position estimator 84.

[0035] Connected to the traction motor 38 is a low resolutionposition/speed sensor 86. Low resolution shaft position and speedsensors, known in the art, can include crankshaft position and speedsensors, camshaft position and speed sensors and transmission positionand speed sensors.

[0036] The low resolution position/speed sensor 86 outputs a shaftposition signal, θ, which is an input to the position estimator 84. Theposition estimator 84 also receives the signals from the inverter 80.The position estimator 84 outputs a modified shaft position signal, θ′.The modified shaft position signal is input into the torque controller82. The torque controller 82 output voltage commands back to theinverter 80.

[0037] As shown, the modified shaft position signal, θ′, is an output ofthe position estimator 84. θ′ can be the position data from the lowresolution position/speed sensor 86. θ′ can also be based on phasecurrent and/or voltage using methods known in the art for sensorlesscontrol. Furthermore, θ′ can be estimated position after correctionusing the low-resolution position/speed sensor 86 input, throughfeedback control or other means known in the art for signal correction.

[0038] For the present invention, sensorless control is the primarycontrol strategy at non-zero traction motor 38 speeds, e.g. speedsgreater than about 10 to about 100 RPM. Typically 50 RPM is a preferredthreshold engine speed. This threshold is selected based on the electricmotor and the accuracy of the sensorless control system. This thresholdcan vary based on the particular application. The sensorless controlestimates position data based on mathematical calculations, such as theback-emf, flux linkage or other means known in the art.

[0039] The present invention can also use the low resolutionposition/speed sensor 86 to revise or correct the sensorless controlestimate to provide a more accurate measurement of rotor position orvelocity. This gives the present invention an ability to compensate forpossible estimation errors caused by measurement noise, faults, machineparameter variations, and to compensate for mechanical transients thatare beyond the bandwidth of the sensorless control algorithm.

[0040] At zero or very low speed, e.g. speeds less than about 50 RPM,sensor-based control is the primary control method using the lowresolution position/speed sensor 86, with the sensorless controlestimate as a possible backup in the event of a system fault or for usein refining the measured signal. To avoid adding additional complexityand/or cost to the system, the low resolution position/speed sensor 86can be one of the low resolution position/speed sensors already presentin the powertrain, such as an engine crank shaft position sensor, enginecamshaft position sensor, or transmission sensor.

[0041] The present invention combines the advantages of sensorlesscontrol and sensor-based control to improve reliability and increaseoperating robustness without significantly increasing system cost. Theinvention can also diagnose potential faults in the electric motorwithout the need for redundant sensors. For example, in the event of afault in the low resolution position/speed sensor 86, the sensorlesscontrol system could function independently. Alternatively, in the eventof a fault in the sensorless control system, the low resolutionposition/speed sensor 86 could function independently.

[0042] The above-described embodiments of the invention are providedpurely for purposes of example. Many other variations, modifications,and applications of the invention may be made.

1. A method for controlling an electric motor, comprising: determiningan electric motor rotational speed; operating said electric motor usinga sensorless control system if said electric motor rotational speed isabove a predetermined threshold; and operating said electric motor usinga sensor based control system if said electric motor rotational speed isbelow said predetermined threshold.
 2. The method according to claim 1,further comprising correcting said sensorless control system with saidsensor based control system.
 3. The method according to claim 1, furthercomprising correcting said sensor based control system with saidsensorless control system.
 4. The method of claim 1, wherein the step ofoperating said electric motor using a sensorless control system if saidelectric motor rotational speed is above a predetermined thresholdcomprises the steps of: determining motor speed and position from aplurality of phase current and phase voltage signals; determining aninverter voltage command from said motor speed and position; anddetermining the plurality of phase current and phase voltage signalsfrom said inverter voltage command.
 5. The method of claim 4, furthercomprising: determining motor speed and position from a position sensor;and correcting said phase current and phase voltage signal determinedmotor speed and position with said position sensor determined motorspeed and position.
 6. The method of claim 1, wherein the step ofoperating said electric motor using a sensor based control system ifsaid electric motor rotational speed is below said predeterminedthreshold step comprises the steps of: determining motor speed andposition from a position sensor; determining an inverter voltage commandfrom said motor speed and position; and determining a plurality of phasecurrent and phase voltage signals from said inverter voltage command. 7.The method of claim 6, further comprising: determining motor speed andposition from a plurality of phase current and phase voltage signals;and correcting said position sensor determined motor speed and positionwith said phase current and phase voltage signal determined motor speedand position.
 8. The method of claim 1, wherein said predeterminedthreshold is about 50 rpm.
 9. The method of claim 1, wherein saidpredetermined threshold is in the range of about 10 rpm to about 100rpm.
 10. A system to control an electric motor comprising: an inverteroperatively connected to said electric motor; a position estimatoroperatively connected to said electric motor and said inverter; a torquecontroller operatively connected to said position estimator and saidinverter; a position sensor operatively connected to said electric motorand said position estimator; a processor for determining a firstelectric motor shaft position based on an output from said inverter;said processor ordered to determine a second electric motor shaftposition based on an output from said position sensor; and saidprocessor programmed to correct said first electric motor shaft positionby using data related to said second electric motor shaft position. 11.The system according to claim 10 wherein said processor is programmed tocorrect said second electric motor shaft position using data related tosaid first electric motor shaft position.
 12. The system according toclaim 10, wherein said position sensor is a low resolution positionsensor.
 13. The system according to claim 10, wherein said positionsensor is an engine crankshaft position sensor.
 14. The system accordingto claim 10, wherein said position sensor is an engine camshaft positionsensor.
 15. The system according to claim 10, wherein said positionsensor is a transmission sensor.
 16. An vehicle comprising: an electricmotor; an inverter operatively connected to said motor; a positionestimator operatively connected to said motor and said inverter; atorque controller operatively connected to said position estimator andsaid inverter; a position sensor operatively connected to said motor andsaid position estimator; means for determining a first electric motorshaft position based on an output from said inverter; means fordetermining a second electric motor shaft position based on an outputfrom said position sensor; and means for correcting said first electricmotor shaft position by said second electric motor shaft positionoutput.
 17. The vehicle according to claim 16, further comprising meansfor correcting said second electric motor shaft position by said firstelectric motor shaft position output.
 18. The vehicle according to claim16, wherein said position sensor is a low resolution position sensor.19. The vehicle according to claim 16, wherein said position sensor isan engine crankshaft position sensor.
 20. The vehicle according to claim16, wherein said position sensor is an engine camshaft position sensor.21. The vehicle according to claim 16, wherein said position sensor is atransmission sensor.
 22. An article of manufacture for controlling anelectric motor, comprising: a computer readable storage device; and acontrol strategy embodied in said computer readable storage device fordirecting a computer to control the steps of determining an electricmotor rotational speed, operating said electric motor using a sensorlesscontrol system if said electric motor rotational speed is above apredetermined threshold, and operating said electric motor using asensor based control system if said electric motor rotational speed isbelow said predetermined threshold.
 23. The article of manufactureaccording to claim 22, wherein said predetermined threshold is about 50rpm.
 24. The article of manufacture according to claim 22, wherein saidpredetermined threshold is in the range of about 10 rpm to about 100rpm.